Tuning CO2 Selective Adsorption over N2 and CH4 in UiO-67 Analogues through Ligand Functionalization

Introducing functional groups into pores of metal–organic frameworks (MOFs) through ligand modification provides an efficacious approach for tuning gas adsorption and separation performances of this type of novel porous material. In this work, two UiO-67 analogues, [Zr6O4(OH)4(FDCA)6] (BUT-10) and [Zr6O4(OH)4(DTDAO)6] (BUT-11), with functionalized pore surfaces and high stability were synthesized from two functional ligands, 9-fluorenone-2,7-dicarboxylic acid (H2FDCA) and dibenzo­[b,d]­thiophene-3,7-dicarboxylic acid 5,5-dioxide (H2DTDAO), respectively, and structurally determined by single-crystal X-ray diffraction. Notwithstanding skeleton bend of the two ligands relative to the linear 4,4′-biphenyldicarboxylic acid in UiO-67, the two MOFs have structures similar to that of UiO-67, with only lowered symmetry in their frameworks. Attributed to these additional functional groups (carbonyl and sulfone, respectively) in the ligands, BUT-10 and -11 show enhanced CO2 adsorption and separation selectivities over N2 and CH4, in spite of decreased pore sizes and surface areas compared with UiO-67. At 298 K and 1 atm, the CO2 uptake is 22.9, 50.6, and 53.5 cm3/g, and the infinite dilution selectivities of CO2/CH4 are 2.7, 5.1, and 9.0 and those of CO2/N2 are 9.4, 18.6, and 31.5 for UiO-67, BUT-10, and BUT-11, respectively. The selectivities of CO2/CH4 and CO2/N2 are thus enhanced 1.9 and 2.0 times in BUT-10 and 3.3 and 3.4 times in BUT-11, respectively, on the basis of UiO-67. The adsorption mechanism of CO2 in BUT-11 has also been explored through computational simulations. The results show that CO2 molecules locate around the sulfone groups in pore surfaces of BUT-11, verifying at the molecular level that sulfone groups significantly increase the affinity toward CO2 molecules of the framework. This provides thus an efficient strategy for the design of CO2 capture materials.通过配体修饰将官能团引入金属有机框架（metal–organic frameworks, MOFs）的孔道中，是调控这类新型多孔材料气体吸附与分离性能的有效途径。本研究以两种功能化配体——9-芴酮-2,7-二羧酸（H₂FDCA）与二苯并[b,d]噻吩-3,7-二羧酸5,5-二氧化物（H₂DTDAO）——为原料，分别合成了两款具有功能化孔表面且稳定性优异的UiO-67类似物：[Zr₆O₄(OH)₄(FDCA)₆]（BUT-10）与[Zr₆O₄(OH)₄(DTDAO)₆]（BUT-11），并通过单晶X射线衍射完成了结构表征。尽管相较于UiO-67中使用的线性4,4'-联苯二甲酸，这两种配体存在骨架弯折，但两款金属有机框架仍保持了与UiO-67相似的框架结构，仅骨架对称性有所降低。得益于配体中额外引入的官能团（分别为羰基与砜基），尽管BUT-10与BUT-11的孔尺寸与比表面积较UiO-67有所下降，但其对CO₂的吸附量以及对N₂、CH₄的分离选择性均得到提升。在298 K、1 atm条件下，UiO-67、BUT-10与BUT-11的CO₂吸附量分别为22.9、50.6与53.5 cm³/g；CO₂/CH₄的无限稀释选择性分别为2.7、5.1与9.0，CO₂/N₂的无限稀释选择性分别为9.4、18.6与31.5。以UiO-67为基准，BUT-10的CO₂/CH₄与CO₂/N₂选择性分别提升了1.9倍与2.0倍，BUT-11则分别提升了3.3倍与3.4倍。本研究还通过计算模拟探索了BUT-11的CO₂吸附机制，结果显示CO₂分子定位于BUT-11孔道表面的砜基附近，从分子层面验证了砜基可显著提升框架对CO₂分子的亲和性，为CO₂捕获材料的设计提供了一种高效策略。